The present invention makes use of devices capable of supporting Josephson tunnelling currents. The basic theoretical explanation of the Josephson effect is given in an article "Possible New Effects for Conductive Tunnelling" by B. B. Josephson, published in Physics Letters, July 1962, pages 251-53. Since then, numerous other publications have disclosed Josephson devices and proposed their application for a variety of functions. A particular advantage of circuits including Josephson devices is their rapid switching speed and their corresponding capabilities to produce pulses having widths in the pico-second range.
A Josephson device is essentially a bi-stable device, in that, in its superconducting or zero voltage state, the device will pass current up to some threshold and so long as the threshold is not exceeded, the device will remain in its zero voltage or superconducting state. The threshold is a function of device parameters as well as any magnetic field in which the device exists. For added purposes of control, Josephson devices are, in some cases, associated with one or more control conductors which, when a current is applied thereto, produce a magnetic field for reducing the maximum threshold current of the Josephson device.
One prior art device, a so-called SQUID, an acronym for superconducting quantum interference device, is simply a Josephson device having two or more control conductors which are arranged so that the switching device may be subjected to a magnetic field which is the net result of the magnetic field produced by the control conductors. When the device switches from its zero voltage or superconducting state to the voltage state, by reason of passing a current through the device which exceeds the threshold, the device will remain in the voltage state until such time as the current through the device is reduced to zero, at which time the device will switch back to the zero voltage state. Significantly, merely reducing the current through the device below the threshold will not switch the device back to a zero voltage state. Thus, the art, in employing Josephson devices, has found it necessary to provide some apparatus or method of operation to reset the Josephson device after it has assumed the voltage state, if it is desired that the device again regains the zero voltage state. Such action is necessary, if, for instance, cyclical operation is desired. In one arrangement, the device is reset by applying to it an alternating current, that is, a current having two different polarities. If the device normally conducts current of one sense, the device can be reset when the current applied to it is in the opposite sense. See, for example, "Clock and Power Distribution System for Josephson Tunnelling Logic Networks" by W. Anacker, appearing in the IBM TDB, Volume 16, Number 10, March 1974, pages 3398-99. On the other hand, the prior art has also proposed the use of special resetting signals and apparatus to effect a similar function, see for example, "Resetting Scheme for Josephson Tunnelling Combinatorial Logic Network" by W. Anacker appearing in the IBM Technical Disclosure Bulletin, Volume 16, Number 10, March 1974, pages 3400-01.
Desirably, of course, the device is arranged in a circuit so that no special resetting pulse signal is required for the reason that the necessity for such signals merely delays useful outputs from the Josephson device, and in effect detracts from the reason why the device is being used, i.e., its speed.
It is therefore an object of the present invention to provide a circuit including Josephson devices which is self-resetting in the presence of a typical pulse input. It is another object of the present invention to provide a circuit including at least one Josephson device, which, when subjected to a pulse input, produces a short output pulse on one transition of the input, and produces no discernible output on the remaining transition which circuit, however, does respond to the complementary transition of the input to reset itself so that on the next one transition of the input, a further output pulse is produced. It is a further object of the present invention to provide a circuit including Josephson devices which, in one embodiment, responds to a positive going transition of an input signal and which, in another embodiment, responds to a negative going transition on the input.
These and other objects of the invention are met by the apparatus of the present invention as will be made clear in the following description.